Apparatus for borehole drilling

ABSTRACT

In deviation borehole drilling with a down-hole motor a plurality of flexible collars is disposed in the drill string between the main stand of weight collars and the deviation assembly. In this way the bending moments of the main stand of weight collars are not transmitted to the down-hole motor and better control of deviation is achieved.

United States Patent 72] Inventor gotw anTrlethswen Edgecornbe 5References Cited UNITED STATES PATENTS [21] Appl. No. 791,913 m1 2222221 21:22: [73] Assignee Rolls-Royce Limited Derby Enghnd 2,805,8409/1957 I 175/76 8 fi Gummy 2.829364 4/1958 175 325 [32] Priority Jan I91968 3,380,543 4/1968 175/73 i 3,476, I l fit a]. [3|] 2915/68Primary,Examiner-MarvinA. Cham ion P Assistant Examiner-Richard ErFavreau V Attorney-Mawhinney & Mawhinney [S4] APPARATUS FOR BOREI-IOLEDRILLING 1 7 ABSTRACT: In deviation borehole drilling with a down-hole[52] U.S.C| 175/76, motor a plurality of flexible collars is disposed inthe drill 175/107 stringbetween the main stand of weight collars and thedevia- [51] In, (I i i E21]; 7/06 tion assembly. In this way the bendingmoments of the main [50] Field ofScarch 175/73, 75, stand of weightcollars are not transmitted to the down-hole motor and better control ofdeviation is achieved.

PATENTED m1 6|97l' Y 3563.323

I INVENTOR Howard Trethewen Edgcombe wan ATTORNE Y5 W W #W APPARATUS FORBOREHOLE DRILLING The present invention relates to apparatus for use indrilling a borehole which deviates from one path to another. Thedrilling of boreholes of thistype is referred to herein as deviationdrilling.

The invention is more particularly concerned with apparatus for use indrilling that section of the borehole which deviates from the said onepath, and which will be referred to hereinafter as the inclined sectionof the borehole. The invention is also particularly, though notexclusively, concerned with a turbodrilling method.

ln deviation drilling, (especially wherein the borehole inclinationdeviates from the vertical by 30 to 40? or more),

the axial component of the weight of the drill collars which producesthe bit thrust is reduced, and that part of the axial load on the drillbit which is due to the weight of the collars in the more vertical partof the drill string, is dissipated by friction due to those parts of thedrill and drill string which rub on the more inclined part of the hole.Thus the load actually applied to the bit, varies in an indeterminatefashion due to the friction at the inclined part of the hole.

Further, in a drill string with a downhole motor, such as a turbodrill,and which has a bentsub to cause deviation, those parts of the drillstring which lie above the bent sub, between the bent sub and the morevertical section of the borehole apply to those parts of the drillstring. which lie below the bent sub, weight bending momentwhich'increases as the deviation from the vertical increases Since thereare usually relatively few parts of the drill string below the-bent sub,the weight bending moment produced above the bent sub, opposing thatproduced below the bent sub, becomes sufficiently great to make controlof the side loads on the drill bit, and of the rate of deviationdifficult, and there is a possibility of exceeding the limit of bendingmoment of the turbodrill.

According to the present invention in deviation borehole drilling with adown-hole motor, a drill string comprises a deviation assembly, ashereinbefore defined, at least one flexible collar connected above the.deviation assembly, and a second collar assembly connected above theflexible collar or collars, the flexible collar or collars beingsufficiently flexible to substantially prevent the second collarassembly from exerting a bending moment on the deviation assembly, butat the same time being capable of transmitting axial loads from thesecond collar assembly to the deviation assembly.

Preferably the flexible collar or collars are provided at their endswith stabilizers.

Also in a preferred embodiment of the invention a ram is provided in thedeviation assembly above the down-hole motor for providing an axialthrust on the drill bit.

According to an embodiment of the present invention, the deviationassembly comprises, from the bottom of the borehole the drill bit, thedown-hole motor, a bent sub and a plurality of drill collars whichprovide the necessary bending moment about the bent sub to produce aside load on the drill bit for deviation.

Use of the flexible collar or collars enables the main stand ofweightcollars to be moved to the less inclined section of the boreholeand thus has an added advantage of allowing more of the weight of themain stand of weight collars to act along the axis of the borehole.

The invention will 'now be described in more detail, merely by way ofexample, with reference to the accompanying drawing which shows avertical section through a partly drilled borehole together with a drillstring assembly of the present invention.

Referring now to the drawing a borehole 1 being drilled, has a straightvertical portion 2, and a curved portion 3 in which the hole deviatesfrom the vertical. The curvature may be, for example 3 per 100 ft.length and the total deviation may be 30 to 40. The curved portion ofthe borehole is shown in the figure with an exaggerated curvature forthe purposes of illustration, the actual length being I000 ft. to 1400ft.

In the borehole 1 is a drill string assembly which comprises a pluralityof hollow drill collars 4 which are attached to a downhole motor, in theform of a turbodrill 5, by means of a bent sub 6 and a ram 7.

The bent sub 6 is a deviation control device and comprises a hollowcylindrical member having an angled end face which provides control ofthe azimuth direction of the drill bit together with the included anglebetween the drill axis and that of the drill collars above it. At thebottom of the borehole 1 is the drill bit 8 which is attached to anddriven by the turbodrill 5. Above the collars 4 is the drill-pipe 10.

The turbodrill 5 is of known type and comprises a turbine (not shown)which is driven by a mud flush which is pumped down from the surfaceinside the drill string, and comes out at the drill bit after passingthrough the turbine. The mud flush then passes back up the borehole l tothe surface passing over the outside of the drill string. The drill bitis driven by the turbine via a shaft 9.

In operation the bent sub contacts the side of the curved portion 3 ofthe borehole at a point K, known as the knee, and those parts of thedrill string which are in the curved portion of the hole produce bendingmoments about K which govern the side load on the drill bit. The ram 7,is of a kind similar to those described in British Patent SpecificationNo. 755207. The interior ofthe ram comprises a hollow central tube andthe mud flush is passed therethrough on its way to the drill turbine. Acylinder surrounds the hollow central tube and a piston is mounted onthe tube for cooperation with the cylinder walls, The hollow centraltube is moveable axially ofthe ram relative to the cylinder. The mudflush passing through the hollow central tube is allowed to communicatethrough the tube with the cylinder on one side of the piston, andthe-.mud flush in the borehole returning to the surface communicatesthrough the cylinder wall with the opposite side of the piston. Thus apressure differential is applied to the piston which is substantiallyequal to the pressure drop of the mud flush in passing through the drillturbine and the drill bit. This thrust will always act axially of theram whatever the inclination of the ram is to the vertical. The ram mayhowever be hydraulically operated.

Above the bent sub are collars 4b, 4c and 4d. Collar 4b is a nonmagneticsurvey collar which contains instruments to detect the borehole headingand inclination. Collars 4c are heavy collars to provide bending momentabout the knee, known as weight collars, and collars 4d are relativelylightweight flexible collars. The collars 4d are provided at their endswith stabilizers 11. Above the flexible collars is the main stand ofdrill collars 4 which comprise, for example, 400 feet of weight collarsand which provide an axial load on the drill bit, and above the weightcollars is the drill pipe 10. The term deviation assembly" is applied tothose parts of the drill string, including the deviation device, whichprovide the forces which govern the side load on the drill bit forcontrolling deviation. la the above-described described example thecollars 40 with a stabilizer 11, the collar 4b, the bent sub and thedrill string below the bent sub collectively form the deviationassembly.

In a turbodrill string assembly without a ram, the drill reliesprincipally on the weight W of the drill collars above it, minus thelift tension L in the drill pipe, for axial thrust T on the bit. This isthe indicated bit weight at the surface. in deviation drilling, as moreand more of the drill collars pass into the curved section of theborehole their axial thrust on the drill decreases for two reasons, (a)less of the weight W of the drill collars acts along their axes asthrust, and (b) more of the collar thrust is dissipated in frictionbetween the collars and the side of the borehole.

The ram, pushing against the weight of the collars above it and thefrictional resistance of the collars and the bent sub to sliding in theborehole, provides a force on the drill bit which, by placing the ramimmediately above the turbodrill, acts axially of the boreholeregardless of the inclination of the borehole. The force provided on thedrill bit by the ram is thus greater than W-L because of the wallfriction.

As the deviation of the borehole from the vertical increases theadvantages of the ram become greater. However, by placing the ram belowthe bent sub, the ram becomes exposed to the bending moments produced bythe drill collars above the bent sub and care must be taken not toexceed the limit of bending moment of the ram. The ram is particularlyuseful at large deviations when the axial component of the weight of thecollars is much reduced. The ram is also particularly useful in thatsection of the borehole below the curved portion where the bendingmoment problem is less severe.

The bending moment problem is overcome by the use ofthe relativelylightweight flexible collars 4d these collars are stabilized at theirends by stabilizers 11 which reduce lateral movements of the ends of thecollars 4d. The collars 4d must besufficiently flexible to preventsubstantial transmission of bending moment but must at the same time becapable of transmitting the axial thrust from the main stand of drillcollars. The collars 4d are of reduced diameter and hence are relativelylightweight so that the bending moment on the drill is substantiallyonly that produced by the collars 4b and 4c and is thus readilycontrolled.

The flexible collars also have the advantage that with their use, themain stand of weight collars can be kept in the more vertical portion ofthe hole so that moreof their weight acts axially and their tendency tofoul on the sides of the borehole is diminished.

The forces acting on the drill bit are the axial thrust T and the sideload S. As above, the thrust T is the ram thrust and it is only a littlemore than the indicated bit weight (W-L). The side load S is produced bythe bending moments acting around K the contact point of the bent sub.

Without the interposed flexible collars the weight bending moment abovethe bent sub increases as the inclined portion of the hole gets longerand more of the drill collars become inclined to the vertical. Deviationcontrol is a difficult process under such conditions, and the bendingmoments may be sufficiently large to damage the drill or the ram. Use ofthe stabilized flexible collars avoids transmission of large bendingmoment from the collars 4 to the deviation assembly and so the deviationbending moment is derived from the clockwise moment of the collars 4band 4c above the bent sub, and the anticlockwise moment of the ram, theturbodrill and the bit below the bent sub, i.e. the deviation assembly.This will be variably affected to a lesser degree by the stabilizedlightweight collars 4d. The ram has to be stiff in order to transmitbending moment and thus a short stroke is preferred.

it will be appreciated that the position of the components of thedeviation assembly may be changed, for example, the ram and the collar41: and may be interchanged. This would place the survey collar nearerthe drill bit, which is preferable, but would cause the ram thrust to beproduced above the bent sub so that the ram thrust would be partiallydissipated by friction on the wall and would act at an angle to thedrill which would be a disadvantage. The main criterion when a ram isused is that the ram should be disposed as close to the drill aspossible.

This embodiment has shown a deviation assembly in which the deviationcontrol device is a bent sub, but it is not meant to be limited to sucha device, since the side load on the bit may be produced by other means.

I claim:

1. A drill string for use in deviation drilling with a down-hole motorcomprising a deviation assembly, at least one flexible provide thenecessar bending moment about the bent sub to produce the require sideload on the dl'lil bit for deviation thereof.

2. A drill string according to claim 1 and in which a ram is provided inthe deviation assembly for providing an axial load on the drill bit.

3. A drill string as claimed in claim l and in which a ram is disposedin the deviation assembly between the down-hole motor and the bent sub.

4. A drill string as claimed in claim I and in which the downhole motoris a turbodrill.

1. A drill string for use in deviation drilling with a down-hole motorcomprising a deviation assembly, at least one flexible collar connectedabove the deviation assembly, a second collar connected above said atleast one flexible collar, said at least one flexible collar beingsufficiently flexible to substantially prevent the second collar fromexerting a bending moment on the deviation assembly, but at the sametime being capable of transmitting axial loads from the second collar tothe deviation assembly, the deviation assembly further comprising, fromits lower end, a drill bit, a down-hole motor, a bent sub, and aplurality of drill collars, which are sufficient to provide thenecessary bending moment about the bent sub to produce the required sideload on the drill bit for deviation thereof.
 2. A drill string accordingto claim 1 and in which a ram is provided in the deviation assembly forproviding an axial load on the drill bit.
 3. A drill string as claimedin claim 1 and in which a ram is disposed in the deviation assemblybetween the down-hole motor and the bent sub.
 4. A drill string asclaimed in claim 1 and in which the down-hole motor is a turbodrill.